Flat batteries that include an exterior can shaped as a cylinder having a bottom and a seal can disposed to cover the opening of the exterior can and connected with the exterior can along its periphery are known. In such a flat battery, as disclosed in, for example, JP-Hei4(1992)-34837A and JP-Sho61(1986)-233965A, a gasket made of resin is disposed at the connection between the exterior can and the seal can to maintain air tightness in the battery and to ensure electrical isolation between the exterior can and the seal can.
JP-Hei4(1992)-34837A and JP-Sho61(1986)-233965A further disclose molding such a gasket on the peripheral wall of the seal can. Particularly, JP-Sho61(1986)-233965A discloses molding, on the peripheral wall of the seal can shaped as a cylinder having a bottom, a gasket that extends from the edge of the opening to the flat portion.